It is well known to prepare isocyanate terminated polyurethane prepolymers from polyether and/or polyester polyols and aromatic diisocyanates. Polyurethane prepolymers are formed by combining an excess of diisocyanate with polyol. One of the —NCO groups of a diisocyanate reacts with one of the OH groups of the polyol, and the other end of the polyol reacts with another diisocyanate. The result is a prepolymer having an isocyanate group on both ends. The prepolymer is therefore a diisocyanate itself, but unlike the original diisocyanate, the prepolymer has a greater molecular weight, a higher viscosity, a lower isocyanate content by weight (% NCO), and a lower vapor pressure.
Furthermore, it is also well known to prepare polyurethane elastomers by chain extending these prepolymers with low molecular weight diols. The resulting polyurethanes have excellent mechanical properties, but are rather hydrophilic, which can limit their utility in certain moisture sensitive applications.
Hydroxyl terminated polyols with very non-polar backbones (e.g., hydroxyl functional polybutadiene) can be used to introduce hydrophobicity into polyurethane elastomers. However, polyols having a polybutadiene backbone, for example, usually have a much higher viscosity than those based on polyether backbone. To reduce the viscosity of hydroxyl-terminated polybutadienes, one can either blend polyether polyols into the polyol mixture or make prepolymers with increased —NCO percentage. The approach is in general not ideal because the final polyurethane products tend to have inferior hydrophobicity.
Thus, there is a need for improved polyurethanes having relatively low viscosity for easier application that are hydrophobic for moisture-sensitive applications.